The invention relates to a gearbox switchable under load. The gear box has an input shaft coupled to at least one further shaft, extending parallel to the input shaft, by pairs of gearwheels to achieve different gear stages. One gearwheel of each pair of gearwheels is rotatively fixedly connected to one of the two shafts. The other gearwheel of each pair of gearwheels is optionally rotatively fixedly connectable to the other of the two shafts by a respective friction coupling.
A gearbox of this type is known from DE 40 07 506 C1 which describes a simply designed gearbox switchable under load. The gearbox is similar in design to manual gearboxes and can be produced at reasonable cost. When starting, the relatively small friction couplings provided for changing gears are subjected to high thermal loads both in the forward and reverse gear. The thermal loads may reach critical values. Because of its length, a gearbox of this type is directly connected to the engine and is less suitable to be arranged transversely in the vehicle.
DE 38 12 327 C2 shows a gearbox switchable under load which is known as a double coupling gearbox. Here, an input shaft is connected to an outer coupling part which accommodates two inner coupling parts connected to separate coaxial shafts. When starting in forward gear, the gearwheels of the two lowest gear stages are engaged. Starting in forward gear is achieved by both coupling parts, and in addition to the switching sleeve of the lowest gear stage, a switching sleeve of the next higher gear stage is actuated at the same time, with the power flow being disconnected in one of the gear stages when the starting speed of the coupling of one of the gear stages has reached the input speed. Contrary thereto, when starting in reverse gear, only one of the coupling halves is loaded. Here, the double coupling is longer than the conventional coupling of a manual gearbox. Because of its design, the gearbox is less suitable to be arranged transversely in a vehicle.
DE 35 46 454 A1 refers to a double coupling gearbox whose characteristics are substantially similar to those described in the previous publication. When starting, both coupling parts are actuated, and the two shafts are connected rigidly by a switching sleeve. When reaching the slip-free condition at the coupling part of the lowest gear stage, the switching sleeve has to be disengaged, thereby generating a switching pressure.
From EP 0 239 553 a short gearbox is known with two intermediate shafts, but it is designed as a manual gearbox.
DE 41 16 989 A1 describes a five or six gear gearbox with three shafts. The gearbox has a short length in the axial direction, but, again, it is designed as a manual gearbox.
U.S. Pat. No. 5,031,473 refers to a five gear gearbox which has a short axial length, but which is also suitable for use as an automatic gearbox, but there is no reference to the arrangement of the reverse gear.
U.S. Pat. No. 5,035,682 describes a six or eight gear gearbox with two reverse gears the latter being actuated by one single multi-plate coupling running in oil. The starting operation is effected by a converter after the respective coupling has been engaged.
To the extent that in the case of the gearboxes of the above type, the vehicle starts in the forward gear by means of the couplings of two gear stages. The fact that the load on the coupling when starting in reverse gear may be just as high as in the case of starting in forward gear has not been taken into account. Prior art gearboxes do not provide a solution for this problem.